Epilepsy in females with mental retardation (EFMR) is a devastating disorder caused by loss-of-function mutations in the protocadherin-19 (PCDH19) gene located on the X-chromosome. The disorder manifests with seizure onset in infancy or early childhood as well as cognitive impairment. This X-linked condition has an unusual inheritance pattern as it only affects females while carrier males are spared. This inheritance pattern has been postulated to arise from random X-inactivation in females where mutant and wild type PCDH19- expressing exhibit abnormal cell-cell interactions to produce the disease phenotype. This cellular interference theory is supported by the single reported case of EFMR in a mosaic male as well as a similar inheritance pattern in another disorder, craniofrontonasal syndrome (CFNS). Although very little is known about the function of PCDH19, it has been shown to be important in neural development. Additionally, PCDH19 strongly interacts with N-cadherin (NCAD) and this complex demonstrates homophilic binding activity. As NCAD also associates with the ?1 subunit of voltage-gated sodium channels (VGSCs), we postulate that PCDH19 mutations disrupt this complex. Our long-term goal is to understand the role of PCDH19 in the brain and how the mutant form leads to epilepsy. Our central hypothesis is that PCDH19 is critical for establishing normal neuronal circuitry. Its disruption during neural development causes abnormal neuronal maturation, patterning, and sodium channel localization through cellular interference of mutant and wild type PCDH19-expressing cells. We will test this hypothesis using EFMR patient-derived induced pluripotent stem cells (iPSCs) with putative loss-of-function mutations in PCDH19. We will differentiate patient-derived iPSCs into cortical excitatory and inhibitory neurons and compare them with respective non-epileptic control iPSC neurons. In Aim 1, we will assess neuronal proliferation, migration, specification, neurite extension and synapse formation. We will determine X-inactivation of mutant vs. wild type PCDH19 and compare mixed cultures expressing both with cultures expressing only mutant or only wild type PCDH19. In Aim 2, we will characterize the electrophysiological properties of EFMR patient-iPSC derived neurons to determine whether there are functional changes with PCDH19 mutations. We will look for differences in intrinsic electrophysiological properties as well as network properties. Additionally, we will determine whether there are changes in subcellular and membrane localization of PCDH19, NCAD, ?1 and VGSCs between EFMR patient and control neurons. We will compare changes in excitatory and inhibitory neurons to look for neuronal subtype-specific effects. Divulging the mechanism behind EFMR will provide a greater understanding of the role PCDH19 plays in neural development and epileptogenesis. Our results may inform future attempts to develop novel anti- epileptic therapies and also allow a greater understanding of the mechanisms of epileptic disorders.